More specifically, the hydroxybenzoic acid polyester resins of this invention are made of parahydroxybenzoic acid, the most useful one among the orth-, meta-, and paraisomers. The full name of this polyester is poly (phenyl p-hydroxybenzoate) or simply polybenzoate. Poly (phenyl p-hydroxybenzoate) is a wholly aromatic linear high polymer. The main chain of this high polymer consists of p-oxybenzoyl repeating units. The structural formula is as follows: ##STR1## n represents degree of polymerization. According to this invention, the value of n can reach 200-500. This high polymer has outstanding heat resistance and can be used at 300.degree. C. for a long period of time, besides it also has a series of other excellent properties such as abrasion resistance, self-lubrication, good heat conduction, dielectric properties, solvent resistance and radiation resistance. It is a special engineering plastic developed in the 1970's and can be used in various industrial and engineering fields wherever the above properties are required. From the viewpoint of the main chain structure, polybenzoate can be regarded as a homopolyester of the bifunctional p-hydroxybenzoic acid, however it was apparent from earlier research data that no high polymer could obtained from homocondensation of p-hydroxybenzoic acid because of the insufficient reactivities of the phenolic hydroxyl and carboxyl groups affected by the benzene ring, and when the temperature is raised to higher than 200.degree. C., p-hydroxybenzoic acid will readily undergo thermal decomposition and decarboxylation. In order to raise the reactivities of hydroxyl and carboxyl groups in p-hydroxybenzoic acid, various approaches have been used such as acetylation of the hydroxyl group, esterification of the carboxy group with aliphatics and acylhalogenation of the carboxyl group before polycondensation. But all these methods could hardly be used in practical production. Some of them only gave product with a low degree of polymerization and insufficient heat-resistance, while others suffered from many side reactions and serious corrosion problems. A formable and machinable polybenzoate with number average molecular weight around 10,000 (degree of polymerization near 100) and good properties was first successfully produced by American Carborundum Co. in 1967. The technical line pursued was esterification of the carboxyl group with phenol to prepare phenyl p-hydroxybenzoate and then polycondensation. The reaction formula is: ##STR2##
The reasons for the success of using phenyl p-hydroxybenzoate as monomer for polycondensation are as follows: the reactivity is significantly raised as the H atom of the carboxyl group in hydroxybenzoic acid is substituted by the benzene ring; the decarboxylation can be suppressed since the carboxylic group is covered owing to the steric hindrance of benzene ring and also due to the high bond energy of the benzene ring. The heat-resistance of the high polymer will be better with hydroxyl group at one end and phenyloxy group at the other. Therefore up to now this technical line has been the principal practical line in this regard. There are several methods for preparing the polycondensation monomer phenyl p-hydroxybenzoate. Those that have been disclosed are:
1. P-hydroxybenzoic acid undergoes ester exchange reaction with phenylacetate (Fr. Pat. 1,568,152) ##STR3##
2. P-hydroxybenzoic acid undergoes similar ester exchange reaction with diphenyl carbonate (Jap. Pat. Appl. Laid Open No. 48-37,355) ##STR4##
3. P-hydroxybenzoic acid undergoes esterification directly with phenol (Jap. Pat. Appl. Laid Open No. 47-39,048) ##STR5##
Because of the very high price of phenylacetate and the need of gaseous HCl catalyst the production cost of method 1 is very high. Diphenylcarbonate used in method 2 is cheaper, nevertheless the input of diphenylcarbonate according to the mole ratio will be significantly increased owing to its high molecular weight, with the consequence that the production cost of method 2 is still very high. The phenol used in method 3 is much cheaper, and very high yield of phenyl p-hydroxybenzoate can conveniently be obtained in the presence of acid catalyst. Besides, the byproduct water is easy to handle, hence method 3 is a more favorable one.
After the formation of phenyl p-hydroxybenzoate, due to its high reactivity its oligomer is readily formed. Therefore, it will be difficult to separate and purify phenyl p-hydroxybenzoate from the reaction mixture by distillation. Recrystallizing or washing method is often used to separate and purify phenyl p-hydroxybenzoate before polycondensation (i.e. two step technique). Both the ester exchange methods of phenylacetate and of diphenyl carbonate had been successful one step techniques and were granted the related patents respectively. The one step technique means the reaction mixture can directly be used for polycondensation and need not to be separated and purified for the monomer. However, because of the difficulty in eliminating thoroughly the acid catalyst and other matters, the monomer mixture obtained from direct esterification of phenol (i.e., method 3) could not be used without purification for polycondensation; and up until 1981, it was still believed that the one step method would appreciably affect the polycondensation rate, and as a result polyester of a high degree of polymerization could hardly be obtained which led to the fact that the two step method was still in use, and with the addition of a special alkali metal compound polyester was produced (Jap. Pat. Appl. Laid Open 56-90-029). Hence up to now nobody has ever been successful in a one step technique by pursuing the above-mentioned technical line (i.e., method 3).
One purpose of this invention is to find out whether polyester with a high degree of polymerization can be produced directly from the monomer phenyl p-hydroxybenzoic acid prepared by reaction of p-hydroxybenzoic acid with phenol without further separation and purification.
Another purpose of this invention is to find out whether phenol, the by-product formed during polycondensation of hydroxyphenyl benzoate, can very simply be reused in the next esterification to form a new monomer phenyl hydroxybenzoate.
The last purpose and also the final goal of this invention is to find out the possibility of establishing a phenol recycling reaction system provided that the above two purposes have been realized. If it is possible, then hydroxybenzoic acid polyester resins of a high degree of polymerization can be obtained directly from the only starting material, hydroxybenzoic acid (i.e., the only reagent which must be replenished). This research problem having strong competitive potentiality in cost is very important and its solution has never been achieved by anyone before.